Moving Beyond Simple Dispatch

Clojure’s print function prints various “sequencey” things as lists. If you wanted my-print to do something similar, you could add a method that dispatched on a collection interface high in the Java inheritance hierarchy, such as Collection:

Now, try various sequences to see that they get a nice print representation:

Perfectionist that you are, you cannot stand that vectors print with rounded braces, unlike their literal square-brace syntax. So add yet another my-print method, this time to handle vectors. Vectors all implement an IPersistentVector, so this should work:

But it doesn’t work. Instead, printing vectors now throws an exception:

The problem is that two dispatch values now match for vectors: Collection and IPersistentVector. Many languages constrain method dispatch to make sure these conflicts never happen, such as by forbidding multiple inheritance. Clojure takes a different approach. You can create conflicts, and you can resolve them with prefer-method:

When you call prefer-method for a multimethod, you tell it to prefer the loved-dispatch value over the dissed-dispatch value whenever there’s a conflict. Since you want the vector version of my-print to trump the collection version, tell the multimethod what you want:

Now, you should be able to route both vectors and other sequences to the correct method implementation:

Many languages create complex rules, or arbitrary limitations, to resolve ambiguities in their systems for dispatching functions. Clojure allows a much simpler approach: just don’t worry about it! If there’s an ambiguity, use prefer-method to resolve it.